[gnumeric] ERF: Fix accuracy of 2-argument version.

[Morten Welinder <mortenw src gnome org>]

commit b2370227582f4da4116d9fe3b880acd9cc39b27e
Author: Morten Welinder <terra gnome org>
Date:   Tue Jun 25 21:00:51 2013 -0400

    ERF: Fix accuracy of 2-argument version.

 NEWS                       |    3 +++
 plugins/fn-eng/ChangeLog   |    4 ++++
 plugins/fn-eng/functions.c |   15 +++++++--------
 3 files changed, 14 insertions(+), 8 deletions(-)
---
diff --git a/NEWS b/NEWS
index 39f1152..dff0766 100644
--- a/NEWS
+++ b/NEWS
@@ -1,5 +1,8 @@
 Gnumeric 1.12.4
 
+Morten:
+       * Improve accuracy for 2-argument ERF.
+
 --------------------------------------------------------------------------
 Gnumeric 1.12.3
 
diff --git a/plugins/fn-eng/ChangeLog b/plugins/fn-eng/ChangeLog
index f302082..773528c 100644
--- a/plugins/fn-eng/ChangeLog
+++ b/plugins/fn-eng/ChangeLog
@@ -1,3 +1,7 @@
+2013-06-25  Morten Welinder  <terra gnome org>
+
+       * functions.c (gnumeric_erf): Fix accuracy for 2-argument version.
+
 2013-06-25  Morten Welinder <terra gnome org>
 
        * Release 1.12.3
diff --git a/plugins/fn-eng/functions.c b/plugins/fn-eng/functions.c
index 963aa4a..3763a94 100644
--- a/plugins/fn-eng/functions.c
+++ b/plugins/fn-eng/functions.c
@@ -1077,6 +1077,7 @@ static GnmFuncHelp const help_erf[] = {
        { GNM_FUNC_HELP_DESCRIPTION, F_("ERF returns 2/sqrt(\xcf\x80)* integral from @{lower} to @{upper} of 
exp(-t*t) dt") },
        { GNM_FUNC_HELP_EXCEL, F_("This function is Excel compatible if two arguments are supplied and 
neither is negative.") },
         { GNM_FUNC_HELP_EXAMPLES, "=ERF(0.4)" },
+        { GNM_FUNC_HELP_EXAMPLES, "=ERF(6,10)" },
         { GNM_FUNC_HELP_EXAMPLES, "=ERF(1.6448536269515/SQRT(2))" },
         { GNM_FUNC_HELP_SEEALSO, "ERFC" },
        { GNM_FUNC_HELP_EXTREF, F_("wiki:en:Error_function") },
@@ -1086,16 +1087,14 @@ static GnmFuncHelp const help_erf[] = {
 static GnmValue *
 gnumeric_erf (GnmFuncEvalInfo *ei, GnmValue const * const *argv)
 {
-       gnm_float ans, lower, upper;
-
-       lower = value_get_as_float (argv[0]);
-       ans = gnm_erf (lower);
+       gnm_float lower = value_get_as_float (argv[0]);
+       gnm_float ans;
 
        if (argv[1]) {
-               /* FIXME: This is suboptimal.  */
-               upper = value_get_as_float (argv[1]);
-               ans = gnm_erf (upper) - ans;
-       }
+               gnm_float upper = value_get_as_float (argv[1]);
+               ans = 2 * pnorm2 (lower * M_SQRT2gnum, upper * M_SQRT2gnum);
+       } else
+               ans = gnm_erf (lower);
 
        return value_new_float (ans);
 }